In the current state of electronic technology, certain devices such as wireless communication devices may require the use of bandpass filters with relatively high out-of-band rejection capability in order to operate effectively. Currently available bandpass filters include, for example, Surface Acoustic Wave (SAW), Bulk Acoustic Wave (BAW) and ceramic filters that are available as discrete components. Alternative approaches have also been proposed and consist of integrating the bandpass filters directly on or into the silicon backend or package substrate. One such solution is the use of embedded circuitry containing a combination of capacitors and inductors to form a bandpass filter as depicted in FIG. 1. In this particular bandpass filter 10, there are five capacitors (C1, C2, C3, C4, and C5) and two coupled inductors (L1 and L2). This arrangement of L1 and L2 is also known as a transformer having a magnetic coupling M1-2 therebetween. The capacitor/inductor combinations, L1C4 and L2C5, form two resonators (i.e, RESONATOR 1, RESONATOR 2) that are each couple to a circuit reference line.
Referring to FIG. 2, which depicts the transmission coefficient—frequency spectrum characteristic curve of the bandpass filter 10 of FIG. 1. The two resonators set the center frequency 22 of the pass frequency band (“passband”) 24 while the combination of capacitor C2 and the mutual inductance between the two inductors L1 and L2 is responsible for the strong rejection of signals at a particular frequency point, which results in the formation of a notch 26 on one side of the passband 24 of the transmission coefficient—frequency spectrum characteristic curve 20. However, such a bandpass filter design produces less than ideal results particularly for applications where strong signal rejection is required on both sides of the passband.
In FIG. 2, the y-axis represents the transmission coefficient in−dB while the x-axis represents the frequency spectrum in GHz. In this illustration, the bandpass filter 10 has a center frequency 22 of the passband 24 around 2.4 GHz. Note that on the left side of the passband 24, the transmission coefficient—frequency spectrum characteristic curve 20 includes a point of strong signal rejection or notch 26 where at the bottom of the notch 26, the transmission coefficient is near −70 dB. Note, however, that there are no notches on the right side of the passband 24. Further, the transmission coefficient above or on the high side of the passband 24 is at most about −16 dB. As a result, the bandpass filter 10 acts somewhat similar to a high pass filter rather than an effective bandpass filter that has high out-of-band rejection on both sides of the passband 24.